This invention relates to a marine propulsion system, and more particularly to such a system incorporating dual counterrotating propellers.
In a marine propulsion system, it is known to employ dual counterrotating propellers for driving a boat. The propellers are oppositely pitched, so that rotation of each propeller provides forward thrust to the boat. Employment of counterrotating propellers offsets torque imbalances which result from the use of a single propeller and improves system efficiency particularly at high speeds and high thrust.
Current practice in designing a dual counterrotating propeller system provides a driving connection between the engine and two concentric propellers which typically employs a fixed drive ratio to each propeller. In such a system, the propellers rotate at equal rotational speeds.
The above-noted design results in operating inefficiencies. For example, the forward one of the two propellers accelerates the water in its path in a helical direction, as all screw propellers do. The amount of helical motion imparted to the water depends on the state of thrust and speed of the propeller at any moment. Whereas the forward propeller operates on relatively undisturbed .water with no rotational movement, the rear propeller runs in the wake of the forward propeller, facing various degrees of rotation and velocity of the water exiting the forward propeller.
To deal with this problem, current practice matches the propeller pitches and diameters and sometimes blade areas to optimize the system efficiency at a selected operating condition. But because boat weight, engine power setting, and operating speed are not within the control of the propeller designer, the propeller match will rarely produce optimum results. One way to refine this approach is shown in Brandt U.S. Pat. No. 4,741,670, which incorporates supercavitating operation in the rear propeller. This system provides improved operation due to the flatter torque to slip relationship of a supercavitating propeller versus a standard propeller. However, the supercavitating propeller efficiency is typically lower than that of a standard propeller at moderate speeds. Furthermore, as operating conditions deviate from the optimal design conditions, some deterioration of efficiency will still occur because of the inability of the rear propeller to change pitch or rotational speed relative to the forward propeller.
The object of this invention is to allow the propellers in a dual propeller installation to deal with the variable, rotational motion and speed of water in the wake of the forward propeller without reference to a selected design condition. As mentioned earlier, a variable adjustment of either pitch or rotational speed of the rear propeller will produce the desired matching of propeller parameters to the operating conditions. Because of the mechanical complexity in the confined space of a propeller hub, the pitch adjustment has been rejected in favor of a rotational speed adjustment which may be accomplished away from the submerged parts of the drive unit.
The essence of the invention is a torque splitting device between the engine output shaft and the propellers which forces a selectable fixed fraction of the engine torque to be transmitted to each propeller regardless of engine power, thrust requirement, or boat speed. The rotational speed of each propeller is allowed to adjust relative to the other propeller through a differential device operationally analagous to an automotive differential gear. Any change in operating conditions at one propeller that causes an increase or decrease of torque allows the forward propeller to slow down or speed up as required by such conditions, causing a corresponding speed-up or slow down in the rotational speed of the second propeller, which results in an increase or decrease of torque in the second propeller until the torque balance is reestablished. This way a precise matching of front and rear propeller parameters is no longer required. Each propeller will provide its assigned share of thrust under all conditions, resulting in optimized system efficiency for a wide range of operating conditions. In addition, the portion of thrust assigned to each propeller is selectable as a result of this invention.
In a typical dual propeller counterrotation marine drive system, the front and rear propellers are each mounted to a propeller shaft, with the respective propeller shafts being rotatably mounted in the lower portion of the marine drive unit housing. The propeller shafts are preferably coaxially mounted within a torpedo formed in the lower portion of the drive unit housing. The torque splitting device of the invention is drivingly interconnected between the propeller shafts and the engine crankshaft. In one embodiment, the marine drive system includes a pair of drive shafts rotatably mounted in the drive unit housing, with each of the drive shafts being drivingly connected to one of the propeller shafts. The torque splitting device is interconnected with the drive shafts so as to provide an adjustment to the relative rotational speed of each drive shaft in response to propeller operating conditions. With this arrangement, the drive shafts extend upwardly above the waterline during boat operation, and the compensating gear means can likewise be disposed above the waterline so as not to effect the frontal area of the submerged portion of the drive unit. A reversing transmission may then also be disposed above the waterline. In another embodiment of the invention, the torque splitting device is housed within the torpedo between an input shaft, driven in response to the engine crankshaft, and the propeller shafts.
In each of the above embodiments, the torque splitting device includes counterrotation drive means which imparts counterrotation to the propeller shafts, and thereby to the propellers. The torque splitting device accomplishes its objective by providing two or more drive pinions mounted to a carrier member, which is adapted to be driven in response to rotation of the engine crankshaft. The drive pinions generally comprise two or more gears rotatably mounted to two or more pins mounted to the carrier member. The two or more drive pinions are interconnected with first and second driven gears, which are drivingly connected to first and second drive shafts. The counterrotation drive means may be located at any satisfactory location in the drive train to ultimately impart counterrotation to the concentric propeller shafts. During normal operation, the drive pinions do not rotate about the pins to which they are mounted. However, when operating conditions vary, the rotational speed adjustment of one or the other of the propellers is transmitted through the gearing system so as to cause the drive pinions to rotate about the pins to which they are mounted. This rotation of the drive pinions is transmitted to the driven gears so as to cause an increase or decrease in the rotational speed thereof, resulting in an increase or decrease in the speed of the propeller driven by such driven gear.